1. Field of the Invention
The present invention relates to a printed wiring board having a core substrate, and insulating resin layers and wiring layers laminated on opposite sides of the core substrate. More particularly, the invention relates to a printed wiring board having a capacitor incorporated therein.
2. Description of the Related Art
Conventionally, a decoupling capacitor is disposed between a ground line and a power supply line for an IC chip in order to eliminate noise. For example, a chip capacitor is mounted on either side of a printed wiring board. FIG. 17 shows a printed wiring board 300 including a core substrate 310, three insulating resin layers 320, 340, and 360 formed on the front side (upper side in FIG. 17) of the core substrate 310, three insulating resin layers 330, 350, and 370 formed on the back side (lower side in FIG. 17) of the core substrate 310, and wiring layers 315, 325, 345, 335, and 355 sandwiched between the core substrate 310 and the insulating resin layers and between the insulating resin layers. By means of solder SL, a chip capacitor CC is mounted on the wiring layer (pads) 355 located on a back side (lower side in FIG. 17) 300B of the printed wiring board 300. Two electrodes CCA and CCB of the chip capacitor CC are connected to the wiring layer (pads) 345 located on a front side (upper side in FIG. 17) 300A of the printed wiring board 300 through through-hole conductors 316 and the wiring layers 315, 325, 335, and 355.
However, many manhours are required to mount such a chip capacitor on a printed wiring board and connect the same to the printed wiring board. Further, since the chip capacitor is disposed on the back side of the printed wiring board or around an IC chip, the distance between the IC chip and the chip capacitor becomes relatively large, permitting entry of noise into a circuit line extending therebetween.
In order to solve the above problem, a capacitor may be incorporated into a printed wiring board and in the vicinity of an IC chip. The capacitor may be formed within the printed wiring board while a portion of an insulating resin layer is used as a dielectric layer therefor.
When a capacitor including a thin dielectric layer sandwiched between electrode layers having a wide area is formed by an insulating resin layer and wiring layers (for example, the insulating resin layer 320 and the wiring layers 315 and 325 in the printed wiring board of FIG. 17), the capacitor is likely to suffer a short circuit, among other problems, causing a significant decrease in the yield of printed wiring boards. When the capacitor is found to be defective, the insulating resin layers and wiring layers that constitute the capacitor must be disposed of together with the core substrate, and this represents a substantial loss.